1. Field of the Invention
This invention relates to a magnetic head assembly and a magnetic recording apparatus.
2. Background Art
In the 1990s, practical application of MR (Magneto-Resistive effect) head and GMR (Giant Magneto-Resistive effect) head has acted as a trigger for recording density and recording capacity of HDD (Hard Disk Drive) to be drastically increased. However, in the 2000s, the problem of thermal fluctuation of the magnetic recording media has been actualized and therefore the speed of the increase of the recording density has temporarily slowed down. Even so, perpendicular magnetic recording, which is fundamentally more advantageous than the longitudinal magnetic recording in high-density recording, has been put into practical use in 2005, and nowadays, the recording density of HDD has been grown by about 40% per year.
Moreover, in the latest recording density verification test, the level of more than 400 Gbits/inch2 has been achieved, and if the recording density steadily develops continuously, it has been anticipated that the recording density of 1 Tbits/inch2 will be realized in about 2012. However, it is thought that realization of such high recording density is not easy even by using the perpendicular magnetic recording scheme because the problem of thermal fluctuation is actualized again.
As a recording scheme that can solve such a problem, “microwave assisted magnetic recording scheme” has been proposed (for example, U.S. Pat. No. 6,011,664). In the microwave assisted magnetic recording scheme, a high-frequency magnetic field in the vicinity of resonant frequency of the magnetic recording medium which is sufficiently higher than recording signal frequency is applied locally to the medium. As a result, the medium resonates and the coercivity (Hc) of the medium of the part to which the high-frequency magnetic field is applied becomes half or less of its original coercivity. By utilizing this effect to superpose the recording magnetic field on high-frequency magnetic field, magnetic recording on the medium with higher coercivity (Hc) and higher magnetic anisotropic energy (Ku) becomes possible. However, in the technique disclosed in U.S. Pat. No. 6,011,664, the high-frequency magnetic field is generated by a coil and therefore it has been difficult to efficiently apply the high-frequency magnetic field to the medium.
Accordingly, as a means for generating the high-frequency magnetic field, techniques for utilizing a spin torque oscillator have been proposed (for example, US Patent Application Publication No. 2005/0023938A1, US Patent Application Publication No. 2005/0219771A1, US Patent Application Publication No. 2008/0019040A1, and IEEE TRANSACTION ON MAGNETICS, VOL. 42, NO. 10, PP. 2670 “Bias-Field-Free Microwave Oscillator Driven by Perpendicularly Polarized Spin Current” by Xiaochun Zhu and lian-Gang Zhu). In the techniques disclosed therein, the spin torque oscillator is composed of a spin injection layer, an intermediate layer, a magnetic layer, and an electrode. When direct current is carried into the spin torque oscillator through the electrode, magnetization of the magnetic layer generates ferromagnetic resonance by spin torque generated by the spin injection layer. As a result, the high-frequency magnetic field is generated from the spin torque oscillator.
The size of the spin torque oscillator is about several tens of nanometers and therefore the generated high-frequency magnetic field is localized in the region of about several ten of nanometers in the vicinity of the spin torque oscillator. Furthermore, by the in-plane component of the high-frequency magnetic field, the perpendicularly magnetized medium can be efficiently resonated and the coercivity of the medium can be drastically lowered. As a result, only in the part in which the recording magnetic field by the main magnetic pole and the high-frequency magnetic field by the spin torque oscillator are superposed, the high-density magnetic recording is performed, and the medium with high coercivity (Hc) and high magnetic anisotropic energy (Ku) can be utilized. Therefore, the problem of thermal fluctuation in the high-density recording can be avoided.
For obtaining a desired effect in the microwave assisted magnetic recording, it is necessary to enhance intensity of the high-frequency magnetic field to be superposed on the recording magnetic field to a certain extent.
For enhancing intensity of high-frequency magnetization, measures of setting Ms·t (Ms is saturation magnetization and t is film thickness) of the magnetic layer used for the spin torque oscillator to be large or measures of shortening the distance between the spin torque oscillator and the recording medium can be thought. However, the former measures are not desirable because the spin torque efficiency falls. Moreover, the latter measures involves technically large difficulty because a floatation amount from the recording medium surface of the head slider on which the magnetic recording head is mounted is currently about 10 nm.